1. Field of the Invention
The present invention relates to a heart stimulator of the type having at least one regulator unit for regulating a variable function in the heart stimulator, a measurement device for measuring an activity-related physiological variable, and a control device for controlling the regulation of the function by the regulator unit on the basis of the physiological variable.
2. Description of the Prior Art
The task of a heart stimulator is to stimulate a defective or dysfunctional heart. In order to provide the patient with an optimum quality of life, the heart stimulator should supply the defective heart with therapy producing a cardiac function as closely resembling the function of a healthy heart as possible. In order to achieve this, the heart stimulator must perform a number of functions, such as determining the amplitude and duration of stimulation pulses, the interval at which stimulation pulses are to be emitted, whether both the atrium and ventricle are to be stimulated, the interval to elapse between an atrial and a ventricular stimulation pulse, etc.
The stimulation interval is a particularly important parameter for the heart stimulator. By changing the stimulation interval, the heart rate can be varied and induced to simulate the rate variations occurring in a healthy heart during e.g., activity or stress. A number of known heart stimulators have been devised which attempt to simulate the response of a healthy heart to activity.
One such prior art heart stimulator is described in U.S. Pat. No. 4,535,774 in which the stroke volume of a heart is determined either by measuring blood flow into the heart or by measuring impedance in the heart. The stroke volume determined in this manner is used to set a stimulation rate to achieve the most optimum cardiac function possible, i.e., to make the amount of blood pumped out the heart each minute sufficient for the body's needs without the heart rate becoming excessively fast. A relationship between stroke volume and heart rate is utilized for this purpose in which stroke volume is assumed to increase with increasing heart rate.
Thus, this known heart stimulator actually measures the heart's stroke volume and then uses changes in this parameter for determining an appropriate stimulation rate which is then imposed on the heart.
The utilization of stroke volume, however, is not an especially suitable way of controlling the heart rate because of the heart's physiological operation and the normal course of a cardiac cycle.
Human blood distributed between two divisions of the circulatory system. The pulmonary division carries blood low in oxygen from the right half of the heart to the lungs for oxygenation and thereafter to the left half of the heart. The systemic division carries oxygenated blood from the left half of the heart, distributes it to body tissue in order to supply same with oxygen, among other things, and returns the blood to the right half of the heart. The veins which return blood to the heart in the systemic division constitute a reservoir for blood and hold most of the body's blood at any given time (about 60% when the body is at rest). The volume of blood pumped each minute in the respective division is referred to as cardiac output. During exertion, stress or the like, cardiac output increases and, accordingly, the flow of blood through arteries and veins also increases. In the arteries the increase of blood flow results in an increase in blood pressure. Pressure does not change much in the veins, however, since venous walls are elastic and stretch when a larger flow of blood must pass. The actual rate of flow, however, does vary.
In principle, the cardiac cycle comprises two phases, a blood-failing phase (diastole) and a blood-emptying phase (systole). Diastole begins with relaxation of atrial heart tissue. Blood then flows into the atria which at the time serve as transient blood reservoirs. Ventricular muscle tissue then relaxes, and the valves between the heart's atria and ventricles open to admit blood into the ventricles. When the ventricles have filled with blood, systole commences with an atrial contraction forcing an additional charge of blood, which can amount to about 1/3 of the total capacity of the ventricles in a healthy person, into the ventricles. When the flow of blood from the atria into the ventricles ceases, the heart valves close, and the ventricles contract to pump blood into the respective circulations divisions. Atrial diastole starts once again during the ventricles' contraction phase.
In principle, the ventricles have a specific maximum volume capacity. Stroke volume can then vary depending on how strongly the ventricles contract, i.e., on the amount of blood remaining in the ventricles at the end of systole. Cardiac output can therefore increase during exertion when both stroke volume and heart rate increase. Most of the increase in cardiac output, however, occurs by means of an increase in heart rate. During excessively fast heart rates, however, the stroke volume can decrease, since the return flow of blood to the heart then does not have time to achieve adequate blood-filling.
When a heart is defective or damaged, the rate at which it beats can be too slow. The return flow of blood to the heart can also be impaired when the atrium and ventricle contract asynchronously. As noted above, the atria contribute about 1/3 of the ventricular blood volume in each heart cycle. Stroke volume thus can be badly affected if the atria and ventricles are not correctly synchronized. A control function which sets a stimulation rate on the basis of changes in stroke volume, as described for the aforementioned known heart stimulator, is therefore inappropriate for persons having a defective cardiac function.
In European Application 0 591 642, a rate-adaptive heart stimulator is described which utilizes the degree of blood-filling of the heart in determining the stimulation point in time. The degree of blood-filling can be determined by measuring blood flow or by measuring impedance in the heart. The flow of blood into the ventricles ceases at the end of the blood-filling phase, and a threshold value can be established which corresponds to an adequate degree of blood-filling. Impedance designates, in the corresponding manner, the degree of filling of the heart.